When analyzing seismic data, the seismic velocity of waves reflected from strata may vary depending on the direction of measurement, thus exhibiting anisotropy. This anisotropy needs to be taken into account, for example, in cases where shales overlie hydrocarbon reservoirs.
Alkhalifah, Geophysics 60: 1550-1566, 1995, shows that a single anisotropy parameter .eta. is sufficient to perform anisotropic time migration if the rocks can be approximated as transversely isotropic with a vertical axis of symmetry. The parameter .eta. needs to be determined if information on strata is to be usefully determined from seismic signals, and in particular from S-waves (Shear waves) which have propagated through an anisotropic medium.
Whilst Alkhalifah proposed a method for determining .eta. using short-offset seismic data from two reflectors of different dip (e.g a fault and a gently dipping reflector), in many cases the layers above the zone of interest, i.e. the overburden, will be flat lying and lack steeply dipping reflectors. In such cases, the method proposed by Alkhalifah becomes difficult to use.
The present invention aims to provide a method for analyzing pre-stack data which allows determination on of the anellipticity relating to a seismic wave propagating in an anisotropic medium, the anellipticity being closely related to the anisotropy parameter .eta..